Among the various types of batteries available at present, lithium ion batteries are attracting attention from the viewpoint of their high energy density. Among these batteries, all-solid-state batteries, in which the electrolytic solution has been replaced with a solid electrolyte, are attracting particular attention. This is because, differing from secondary batteries using an electrolytic solution, since all-solid-state batteries do not use an electrolytic solution, there is no degradation of the electrolytic solution caused by overcharging and these batteries have high cycling characteristics and high energy density.
Olivine-type positive electrode active materials are known to be used for the positive electrode active materials used in lithium ion batteries. Olivine-type positive electrode active materials have a more stable structure and higher cycling characteristics in comparison with other positive electrode active materials. Consequently, research has recently been conducted on all-solid-state batteries using olivine-type positive electrode active materials.
Patent Document 1 and 2 are examples of the patent literature relating to all-solid-state batteries using an olivine-type positive electrode active material.
In Patent Document 1, an electron conduction path is formed in a positive electrode active material layer by bonding filamentous carbon to an olivine-type positive electrode active material.
Patent Document 2 discloses a technology that uses a slurry containing a positive electrode active material and a binder in a method for producing a sulfide solid-state battery. Patent Document 2 also lists an olivine-type positive electrode active material as an example of a positive electrode active material.
In addition, research has been conducted on a technology for making the capacity ratio between a positive electrode active material layer and a negative electrode active material layer to be a specific ratio in a solution-based battery and all-solid-state battery.
Patent Document 3 discloses a battery system in which a solution-based battery and an all-solid-state battery are connected in parallel, wherein the ratio of the capacity of a negative electrode active material layer to the capacity of a positive electrode active material layer in the solution-based battery is made to be larger than the ratio of the capacity of a negative electrode active material layer to the capacity of a positive electrode active material layer in the all-solid-state battery. As a result, the all-solid-state battery shorts out prior to overcharging, thereby preventing the solution-based all-solid-state battery from shorting out.
Patent Document 4 discloses a solution-based battery that uses an electrolytic solution obtained by dissolving a lithium salt in water, wherein the attainable potential of the positive electrode active material layer during charging is lowered by making the ratio of the capacity of a negative electrode active material layer to the capacity of a positive electrode active material layer to be 0.5 to 1.0.
Patent Document 5 discloses an all-solid-state battery designed so that the capacity of a positive electrode active material layer is lower than the capacity of a negative electrode active material layer. As a result, lithium metal is inhibited from being deposited on the negative electrode active material layer during charging.